Question: The scale on a map is 2cm : 10km. If two cities are 20cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 2cm on the map corresponds to an actual distance of 10km. A distance of 20cm on the map is the same as 10 $\cdot$ 2cm. The actual distance between the two cities is 10 $\cdot$ 10km, or 100km.